1. Field of the Invention
The present invention generally relates to multi-piece golf balls having a solid core of at least one layer and cover of at least one layer. A thermoplastic ionomer composition is used to form at least one of the core layers. Preferably, the ball contains a dual-core having an inner core and surrounding outer core layer or a multi-layered core having an inner core, intermediate core layer, and outer core layer. Low modulus and high modulus ionomer compositions may be used in the core structure. The resulting ball has high resiliency and rebounding properties.
2. Brief Review of the Related Art
Golf ball manufacturers are continuously looking to improve the properties and playing performance of golf balls. One way for improving the quality of golf balls is by designing and developing new ball materials and structures. Multi-piece, solid golf balls having a solid inner core protected by a cover are used today. Normally, the inner core is made of a natural or synthetic rubber material such as, for example, styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene); and the outer cover is made of a durable material such as, for example, ethylene acid copolymer ionomer resins, polyamides, polyesters, polyurethanes, or polyureas. The core and cover may be single or multi-layered. Intermediate (casing) layers may be disposed between the core and cover layers to form four-piece and five-piece balls and the like. Today, the industry is interested, among other things, in making balls that can rebound faster, retain more total energy when struck with a club, and have longer flight distance.
In general, the rebounding performance of a golf ball is based on its initial velocity after being struck by the face of a golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “coefficient of restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under these conditions. Balls with a higher rebound velocity have a higher COR value. To improve the resiliency and rebounding performance of the golf ball, the industry has focused primarily on the material and construction of the ball's core.
Golf balls having single-layered and multi-layered cores have been developed. For example, Bulpett et al, US Patent Application Publication US 2009/0227394 discloses multi-layered core construction comprising: a) an inner core formed from a first thermoset rubber composition; b) an intermediate core layer formed from a partially-neutralized or highly-neutralized ionomer composition; and c) an outer core formed from a second thermoset rubber composition. A cover layer having a thickness of about 0.01 to 0.05 inches and a surface hardness of about 60 Shore D or less is formed around the core.
Sullivan et al., US Patent Application Publication No. US 2009/0017940 discloses golf balls having a dual-core and a single-layered cover. The dual-core includes an inner core formed from a rubber composition and an outer core layer formed from a highly neutralized polymer (HNP) composition comprising an ethylene acid copolymer. In the HNP composition, at least 80% of all acid groups are neutralized. The inner core has an outer surface hardness of less than 80 Shore C; the outer core layer has an outer surface hardness of 56 Shore D or greater; and the cover layer has a material hardness of 60 Shore D or less.
Sullivan et al., U.S. Pat. Nos. 7,357,736 and 7,211,008 disclose golf balls comprising: a) an inner core layer formed from a diene rubber composition; (b) an outer core layer formed from a high modulus highly neutralized polymer (HNP) comprising a highly neutralized ethylene/(meth)acrylic acid copolymer having a modulus of from 45,000 psi to 150,000 psi; (c) an intermediate core layer disposed between the inner core layer and the outer core layer and formed from a low modulus HNP composition comprising a highly neutralized ethylene/(meth)acrylic acid/alkyl (meth)acrylate copolymer having a modulus of from 1,000 psi to 50,000 psi. In the HNP compositions, at least 80% of all acid groups are neutralized.
Although some conventional multi-layered core constructions are generally effective in providing high resiliency golf balls, there is a continuing need for improved core constructions in golf balls. Particularly, it would be desirable to have multi-layered core constructions that could be manufactured efficiently, have relatively low material costs, and provide the ball with high quality and performance properties. In particular, the core should provide the ball with good flight distance along with a comfortable and soft feel. The present invention provides core constructions and golf balls having such properties as well as other advantageous features, and benefits.